The invention relates to semiconductor device fabrication and, more particularly, to techniques for capping interlevel dielectric (ILD) layers of interconnect structures such as damascene interconnect structures.
An integrated circuit (IC) device (also referred to as a semiconductor chip) can contain millions of transistors and other circuit elements that are fabricated on a single silicon crystal substrate (wafer). For the IC device to be functional, a complex network of signal paths will normally be routed to connect the circuit elements distributed on the surface of the device. Efficient routing of these signals across the device can become more difficult as the complexity and number of circuit elements are increased. Thus, the formation of multi-level or multi-layered interconnection schemes such as, for example, dual damascene wiring (interconnect) structures, have become more desirable due to their efficacy in providing high speed signal routing patterns between large numbers of transistors and other circuit elements on a complex IC.
Damascene Techniques
Generally, damascene techniques involve depositing an interlevel dielectric (ILD) layer, such as silicon dioxide (SiO2; also referred to simply as “oxide”), forming an opening in the ILD, overfilling the opening with a metal such as copper (Cu), and removing excess metal using chemical mechanical polishing (CMP), resulting in a planarized interconnect structure. This results in a single “wiring level” (or “interconnect level”) of an overall “interconnect structure” having many wiring levels. The opening in the interconnect level may be a trench running essentially parallel to the surface of the integrated circuit (IC) chip, and a filled trench is referred to as a “wire” or a “line”. A wire is used to route signals from a first location on the wafer to a second location remote from the first location. The trench for forming the wire may extend only partially (e.g., halfway) into the thickness of the ILD, from the top surface thereof. In a sense, the copper conductor is “embedded” in the ILD.
Alternatively, an opening in the interconnect level may be a via, extending perpendicular to the surface of IC completely through the ILD for connecting an overlying wire of a higher wiring level or of the present wiring level (in dual damascene, described below) to an underlying wire of a lower wiring level. A filled via is typically simply referred to as a “via”, and sometimes as a “plug” particularly when connecting to an underlying first metallization (M1) or to an element of an underlying MOS (metal oxide semiconductor) structure. Vias and wires are both referred to herein as “conductors”, since their raison d'etre is conducting electrical signals.
In “dual” damascene techniques, the opening in the ILD comprises a lower contact or via hole portion in communication with an upper trench portion, and both the via and the trench portions are simultaneously filled.
Presently, interconnect structures formed on an integrated circuit chip consist of at least about 2 to 8 wiring levels fabricated at a minimum lithographic feature size (currently approximately 0.25 μm (microns) designated about 1× (referred to as “thinwires”) and above these levels are about 2 to 4 wiring levels fabricated at a width equal to about 2× and/or about 4× the minimum width of the thinwires (referred to as “fatwires”). A typical width for a via is about 130 nm (nanometers), and it is common to have redundant vias effecting connections between overlying and underlying wires. 1 μm (micron)=1000 nm (nanometers).
Copper (Cu) and Cu alloys have received considerable attention as a candidate for replacing aluminum (Al) and Al alloys in interconnect metallizations. Cu is relatively inexpensive, easy to process, and has a lower resistivity than Al. In addition, Cu has improved electrical properties vis-a-vis tungsten (W), making Cu a desirable metal for use as a conductive plug as well as conductive wiring. As use herein, “Cu” is intended to encompass high purity elemental copper as well as Cu-based alloys, such as Cu alloys containing minor amounts of tin, zinc, maganese, titanium, magnesium, germanium, aluminum and silicon.
Due to Cu diffusion through interdielectric layer (ILD) materials, such as silicon dioxide, Cu interconnect structures should be encapsulated by a diffusion barrier layer (or “liner”). Conventional practices comprise forming a damascene opening in an ILD, and depositing a barrier layer such as TaN, lining the sidewalls and bottom of the opening in the ILD prior to depositing the Cu for the via or wire.
Typical diffusion barrier layer metals include tantalum (Ta), tantalum nitride (TaN), titanium nitride (TiN), titanium-tungsten (TiW), tungsten (W), tungsten nitride (WN), Ti—TiN, titanium silicon nitride (TiSiN), tungsten silicon nitride (WSiN), tantalum silicon nitride (TaSiN) and silicon nitride for encapsulating Cu. The advantage of using such barrier layer materials to encapsulate Cu is not limited to the interface between Cu and the dielectric interlayer, but includes interfaces between the Cu and other metals as well.
The upper surface of any Cu conductor (typically a wire, since a via, by definition, will always be in contact with a bottom surface of an overlying conductor) must also be protected, such as against oxidation. To cap the upper surface of the copper interconnection, a “capping layer” of a dielectric material such as silicon nitride (Si3N4, also simply referred to as “nitride”) is typically employed. The capping layer is also referred to as a “passivation layer”. Often the passivation layer must also function as an etch stop layer during subsequent processing, however materials which perform best as etch stop layers often do not perform best as passivation layers. For example, silicon oxynitride, SiON, is useful as an etch stop layer but it is less desirable as a passivation layer because of delamination which can occur between copper and silicon oxynitride. Silicon nitride, “SiN”, avoids the delamination problem, and is a preferred passivation material, but is less desirable as an etch stop layer.
FIG. 1 illustrates a conventional BEOL (back end of line) interconnect structure 100 utilizing copper metallization, the barrier layers and the protective cap layers described above. The illustrated interconnect structure 100 comprises a first interconnect level 110 and a second interconnect level 130 and is shown (by way of example) as being formed on a substrate 102 such as a semiconductor wafer comprising a plurality of logic circuit elements such as transistors. A single “generic” contact area 104 is illustrated in the substrate 102 and is, for example, an electrode formed on a source or drain region of a MOSFET (metal oxide semiconductor, field effect transistor).
It should clearly be understood that FIG. 1 illustrates but an extremely small (microscopic) portion of an integrated circuit (IC) device, let alone a semiconductor wafer comprising a large plurality of such devices. For example, what is shown may have a width of only a few microns (μm) of a semiconductor wafer having a diameter of several inches. Also, in “real life” things are not so neat and clean, rectilinear and uniform as shown. However, for one of ordinary skill in the art to which the invention most nearly pertains, this and other figures presented in this patent application will be very useful, when taken in context of the associated descriptive text, for understanding the invention.
The first interconnect level 110 comprises an interlevel dielectric layer (ILD) 112, such as oxide which is prepared by a chemical vapor deposition (CVD) process and having an exemplary thickness of 8000 to 10,000 angstroms, or 800–1000 nm (nanometer). (1 nm=10 angstroms)
In a “via first” damascene process, a via 116 is formed, such as by reactive ion etching (RIE) and extends to the bottom surface of the ILD 112 (in this case, to the electrode 104 on the substrate 102). Then, a trench 114 is formed (also using RIE) extending into the ILD 112 from the top (as viewed) surface thereof, and having a depth of nominally several (e.g., 4–5) thousand angstroms.
The trench 114 and via 116 comprise the “opening” in the ILD 112. A barrier layer 118, such as TaN, is deposited such as by sputtering or CVD so that it lines the sidewalls and bottom of the opening in the ILD 112. A typical thickness for the barrier layer 118 is between 600 and 1,000 angstroms, and metal for the barrier layer will also deposit on the top surface of the ILD 112.
Copper (Cu) 120 is then deposited into the lined opening, and will overfill the opening. Next, chemical mechanical polishing (CMP) is performed to remove excess barrier material and copper from the surface of the ILD 112, leaving a planarized surface for subsequent semiconductor fabrication processes to be performed. The copper 120 forms a wire (or line) in the trench 114 and a plug (or via) in the via 116. Because the top surface of the wire is exposed, a capping layer 122 such as nitride is deposited by CVD on the surface of the wire, and has an exemplary thickness of 500 angstroms (50 nm). This capping layer 122 is eventually patterned, after the next level of dielectric (132) is deposited, using photoresist (not shown) and conventional photolithographic techniques, to have an opening 124 for allowing a via (136) of a subsequent (higher) wiring level (130) to make contact with the wire 120 in the trench 114.
The second interconnect level 130 is formed atop the first interconnect level 110 and is essentially identical to the first interconnect level 110. Both levels 110 and 130 are shown as being formed by a dual damascene process. The second interconnect level 130 comprises an interlevel dielectric layer (ILD) 112, such as oxide. A via 136 (compare 116) is formed, using RIE, extending to the bottom surface of the ILD 132. A trench 134 (compare 114) is formed in the ILD 132. The trench 134 and via 136 comprise the “opening” in the ILD 132.
After the via 136 is formed, the capping layer 122 is opened up 124 so that so that metal 140 filling the via 136 of the second interconnect level 130 can make electrical contact with the metal 120 filling the trench 114 of the underlying, first interconnect level 110.
A barrier layer 138, such as TaN, is deposited so that it lines the sidewalls and bottom of the opening in the ILD 132, and is processed as described hereinabove. Copper (Cu) 140 is deposited into the lined opening, and is processed as described hereinabove. The copper 140 forms a wire (or line) in the trench 134 and a plug (or via) in the via 136. A capping layer 142 such as nitride is deposited on the surface of the ILD 132 and the wire 140. If necessary, the capping layer 142 is will be opened up (compare 124) to allow a via (not shown) of a subsequent (higher) wiring level (not shown) to make contact with the wire 140 in the trench 134.
The dual damascene interconnect structure 100 shown in FIG. 1 is fabricated utilizing conventional damascene processing steps well known to those skilled in the art. Since such techniques are well known and are not critical for understanding the present invention, a detailed discussion of the same is not given herein. It will be understood that various steps and materials have been omitted, for illustrative clarity, such as seed layers, adhesion layers, cleaning steps and the like.
Dielectric layers 112 and 132 may be the same or different insulative inorganic or organic material. Suitable dielectrics include, but are not limited to: SiO2, carbon rich oxides, fluorinated SiO2, polyimides, diamond, diamond-like carbon, silicon polymers, paralyene polymers, fluorinated diamond-like carbon and other like dielectric compounds.
Low-k Dielectric Materials
Semiconductor devices are typically joined together to form useful circuits using interconnect structures comprising conductive materials (e.g., metal lines) such as copper (Cu) or aluminum (Al) and dielectric materials such as silicon dioxide (SiO2). The speed of these interconnect structures can be roughly assumed to be inversely proportional to the product of the line resistance (R), and the capacitance (C) between lines. Line resistance can be reduced (hence, speed increased) by using copper (Cu) instead of aluminum (Al). To further reduce the delay and increase the speed, it is desirable to reduce the capacitance (C). One way in which this can be done is by reducing the dielectric constant “k”, of the dielectric material in the interlevel dielectric layers (ILDs). Thus, there is considerable interest in developing “low-k” materials as well as deposition methods for them that are compatible with integrated circuit technology.
A common dielectric material for use in an interlevel dielectric layer (ILD) is silicon dioxide (SiO2, also referred to simply as “oxide”). Oxide has a dielectric constant k of at least 3.85, typically 4.1–4.3, or higher. Air has a dielectric constant k of approximately 1.0. By definition, a vacuum has a dielectric constant k of 1.0.
A variety of low-k dielectric materials are known, and are typically defined as materials having a dielectric constant k less than 3.85—in other words, less than that of oxide. Sometimes, materials having k<2.5 are referred to as “ultralow-k”. These low-k and ultralow-k dielectric materials can generally be characterized by their composition and/or by the way in which they typically are deposited.
Deposition is a process whereby a film of either electrically insulating (dielectric) or electrically conductive material is deposited on the surface of a semiconductor wafer. Chemical Vapor Deposition (CVD) is used to deposit both dielectric and conductive films via a chemical reaction that occurs between various gases in a reaction chamber. Plasma enhanced Chemical Vapor Deposition (PECVD) uses an inductively coupled plasma to generate different ionic and atomic species during the deposition process. PECVD typically results in a low temperature deposition compared to the corresponding thermal CVD process. Spin-on deposition is used to deposit materials such as photoresist, and can also be used to deposit dielectric materials. A wafer is coated with material in liquid form, then spun at speeds up to 6000 rpm, during which the liquid is uniformly distributed on the surface by centrifugal forces, followed by a low temperature bake which solidifies the material.
Examples of spin-on low-k materials include:                BCB (divinylsiloxane bisbenzocyclobutene), sold by Dow Chemical.        SiLK™, an organic polymer with k=2.65, similar to BCB, sold by Dow Chemical.        NANOGLASS™, an inorganic porous polymer with k=2.2, sold by Honeywell        FLARE 2.0™ dielectric, an organic low-k poly(arylene)ether available from Allied Signal, Advanced Microelectronic Materials, Sunnyvale, Calif.        Inorganic materials such as spin-on glass (SOG), fluorinated silicon glass (FSG) and, particularly, methyl-doped porous silica which is referred to by practitioners of the art as black diamond, or BD.        Organo-silicate materials, such as JSR LKD 5109 (a spin-on material, Japan Synthetic Rubber)        Organic polymers (fluorinated or non-fluorinated), inorganic polymers (nonporous), inorganic-organic hybrids, or porous materials (xerogels or aerogels).        Materials in the parylene family of polymers, the polynapthalene family of polymers, or polytetrafluoroethylene.        
Examples of low-k Chemical Vapor Deposition (CVD) and Plasma Enhanced CVD (PECVD) low-k materials include:                Black Diamond™, a organosilicon glass (OSG) which is a Si—O—C—H type of material, with a dielectric constant k of 2.7 to 3.0 (e.g., 2.9), sold by Applied Materials Inc.        CORAL™, also an organosilicon glass (OSG) which is a Si—O—C—H type of material, with k of 2.7–3.0, sold by Novellus Systems, Inc.        fluorinated SiO2 glass (FSG), and        diamond like carbon or fluorine-doped diamond like carbon (amorphous C:F).        
It is also known that pores in dielectric materials can lower the dielectric constant. Low-k dielectric materials can typically be deposited ab initio either with or without pores, depending on process conditions. Since air has a near 1 dielectric constant, porous films exhibit reduced dielectric constants than the base material in which they are developed. Generally, it is the spin-on materials (e.g., SiLK, NANOGLASS) materials that exhibit a high degree of porosity. The PECVD materials generally do not exhibit such high degree of porosity, due to the method of deposition. As a result it is very difficult to prepare a CVD film with a k value <2.5. For low-k dielectric materials having pores, it is important that an additional layer or film overlies the porous dielectric layer to act as a moisture barrier for the porous dielectric layer.
Capping Layer
Due to the need for low temperature processing after copper deposition, capping layers (also referred to as “cap layers”) are typically deposited at temperatures below 450° C. Accordingly, capping layer deposition is typically performed using plasma-enhanced chemical vapor deposition (PE CVD) or high density plasma chemical vapor deposition (HDP CVD) wherein the deposition temperature generally ranges from about 200° C. to about 500° C.
PE CVD films have been used for many other applications in semiconductor device manufacturing. However, in using a cap layer such as silicon nitride for copper interconnects, conventional PE CVD silicon nitride films can create reliability problems. HDP silicon nitride has been shown to provide improved reliability vs. PE CVD silicon nitride (or “PE nitride”).
HDP CVD films such as silicon nitride provide superior electromigration protection, as compared to PE CVD films, because HDP CVD films more readily stop the movement of copper atoms along the interconnect surface in the cap layer. However, in a conventional HDP deposition process, a seam is formed in the HDP CVD capping layer over topography, and a crack in the capping layer often develops at this seam due to stress within the structure. If the crack develops in a portion of the capping layer overlying a copper conductor, the copper conductor may be readily exposed to moisture and other sources of oxygen. If the crack develops in a portion of the capping layer overlying the ILD, the copper conductor may be exposed to moisture diffusing through the ILD. In the latter case, the seam is of relatively minor concern in interconnect structures utilizing silicon dioxide as the ILD material, because the rate of moisture diffusion through silicon dioxide is very low. However, in interconnect structures utilizing low-k polymeric thermoset dielectric materials such as SiLK™, this seam is of greater concern, because the rate of moisture diffusion through most spin-on and CVD low-k materials is relatively high.
Moreover, any crack in the capping layer may lead to copper diffusion into the ILD through the seam. As a result of this copper diffusion, a copper nodule may form under the capping layer through the seams. This copper nodule may lead to leakage between adjacent interconnect lines.
HDP nitride is a common material for use as the capping layer over ILD as it exhibits excellent control over electromigration of underlying copper (Cu). However, HDP nitride has been found to exhibit seams which allow ingress of reactants (e.g., oxygen) into the underlying copper (Cu).
Because of the problem of seams in the HDP nitride, placement of an additional layer of material over the HDP nitride, to seal the seams, such as UV nitride (ultra-violet nitride; “UVN”) or other plasma nitride has been suggested. (UVN is similar to and is sometimes referred to as PE CVD, or simply PE nitride.) However, PE nitride is not selective to oxide and will be compromised during via RIE. Another approach is to change the selectivity of the RIE, but this may increase the probability of underetched vias. Another approach is the use of alternate cap nitride film, but this loses the electromigration benefit of HDP nitride atop Cu.
UV nitride (UVN) is a particular form of PE CVD (PE) nitride deposited under certain select conditions; however, all PE nitrides can be distinguished from high density plasma (HDP) nitrides by virtue of process pressure and hardware used in the delivery of the HD plasma. PE nitrides are typically deposited in the range of a nominal 1 Torr pressure regime vs. a pressure of a few milliTorr used in HDP CVD. As a general proposition, with respect to a broad process window for RIE, PE nitrides are easier to etch than HDP nitride.